Unit: OPTICS
Year: 1992 Question#: 92
Question: As a teacher showed slides by projecting them on a fixed screen, a student complained that the image was too small. The teacher enlarged the image by moving the projector away from the screen, but the image blurred. The image should then have been brought into focus by
(1 ) moving the lens closer to the slide
(2 ) moving the lens away from the slide
(3 ) decreasing the amount of light in the room
(4 ) increasing the power of the projector lamp
What is this question really asking?
Explanation by: Smar T.
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Answer 1
moving the lens closer to the slide
CORRECT
The image is real, since it can be projected onto a screen, and therefore, the lens must be converging (only converging lenses produce real images). Since the screen has been moved further from the projector, the image distance must also be greater in order for the image to be in focus. With a converging lens, the only way to get a larger image distance is by decreasing the object distance (from the lens), determined by the equation
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Answer 2
moving the lens away from the slide
INCORRECT
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Answer 3
decreasing the amount of light in the room
INCORRECT
light INTENSITY has nothing to do with image distance and object distance; it effects brightness of the image only. return to top
Answer 4
increasing the power of the projector lamp
INCORRECT
Again, light intensity has nothing to do with image/object distance. (see explanation for choice (3)) return to top
What's this question really asking?
What kind of lense produces a real image? Knowing this, how does image distance vary with object distance? return to top
